Glass has been widely used as a material for conventional containers of seasonings, oils, juices, carbonated drinks, beer, Japanese sake, cosmetics, detergents etc. Since the production cost of glass containers is high, empty containers already used have usually been collected and reused repeatedly. Furthermore, the glass containers have disadvantages such as their fragility and inconvenience of handling in addition to a high cost of transportation due to their heaviness.
Because of these drawbacks, glass containers have recently been replaced rapidly by plastic containers. Various kinds of plastics are employed as the container materials according to the sort of contents to be filled and the purpose of usage. Among these plastic materials, saturated polyester resins such as polyethylene terephthalates, being excellent in mechanical strength, heat resistance, transparency and gas barrier properties, are adopted as the materials of the containers for juices, cooling drinks, carbonated drinks, seasonings, detergents, cosmetics and the like.
Recently, the thickness of the bottles made of saturated polyesters such as polyethylene terephthalates tends to be thinner to lower the material cost of bottles. However, an excessive decrease in the thickness of bottles lowers the mechanical strength and sometimes causes breakage of the bottles filled with carbonated drinks, particularly, in the summer time.
The breakage of bottles may not only hurt the commodity images but also cause accidents. Therefore, it is very important in practical use to prevent the breakage of bottles.
The present inventors have studied earnestly the cause of the breakage of bottles made of saturated polyesters and found that most bottles have broken vertically at their upper shoulder region of 5-20 mm from the lower end of the mouth (the neck ring) and the breakage frequency has increased at the higher temperatures and the higher humidities. They also have found by further study based on the above information that the breakage of the saturated polyester bottles mentioned above is scarcely responsible for the chemical structures of the saturated polyester and the breakage can be prevented almost completely, providing the saturated polyester resin at the upper shoulder region of bottles has specific physical properties, and have finally accomplished the present invention.